Servo system



Noy.r2l, 1944. N. I EsNlcK I 2,363,342

SERVO SYSTEM Filed July 29, 1942 A F'yl HMPLIF'IER HND POWER j V T 'ro l a-c .SUPPLY MAI lllll "v Gttorneg provided to supply the plate and grid bias volt# ages for the amplifier 21.

'Ihe operation of the system is as follows: As long as the control shaft I3 and the output shaft 1- are stationary land in positional agreement, no voltage appears at the secondary of the transformer II, and hence there is no input to the control device I1. When either shaft is displaced,

. however, there is produced a voltage related in magnitude and phase to the extent and direction of the displacement. This voltage is compared in phase with the line voltage in thedevice 25,

which produces a yunidirectional voltage having a polarity .related to the direction of the displacement and a magnitude related to the extent of the displacement. This voltage is amplified and used to control the power stage 29 to energize the motor I so as to drive the output shaft 1, and with To reduce the eiectivefeedback yat high motor speeds, the resistor I9 is connected in series with the rectifier circuit. At motor speeds greater than a predetermined value. the switch 5 operates to insert the resistor I9 and provide decreased feedback.'

Thus the invention has been described as an improved stabilizing arrangement for servo systems. A velocity signal is derived and is applied to oppose the effect of the displacement signal to reduce the sensitivity of the system with increase in speed of the driving motor up to a predetermined speed. Control of the feedback level in response to thev motor speed offers the advantage of automatic adjustment of thev feedback delay time to meet varying conditions of low speed and high speed operation.

'I claim as my invention:

1. A servo system including an input shaft, an output shaft, means for deriving a control signal related to the difference in the angular positions of saidfshafts, means responsive to said signal to control a motor connected to said output shaft so as to drive said output shaft toward positional agreement with said input shaft, means responsive to the counterelectromotive force of said motor to produce a second signal tending to oppose said control signal, and means responsive to the speed of said motor to vary the magnitude of said second signal.

2. A positional control system comprising a control shaft, a controlled shaft, means for deriving a unidirectional control voltage related to the difference in the angular positions of said shafts, an electric motor mechanically connected to said controlled shaft, means for deriving a unidirectional feedback voltage related in magnitude to the counter electromotive force of said motor and opposing saidv control voltage, means responsive to the combined effects of said control voltage and said feedback voltage to cause said motor to operate to drive said output shaft toward positional agreement with said input shaft,

3. In a positional control for driving a controlled shaft toward angular positional agreement with a control shaft, means for producing and receiving a signal. a reversi-ble power motor for driving the controlled shaft. a thermionic tube circuit for controlling said motor in accordance with said signal, feedback means for producing a direct current feedback in said circuit for rendering said circuit less sensitive after said motor has started, and means responsive to `the speed of said motor for controlling the effect of said feedback.

4. A remote control system comprising a motor, electron discharge means arranged to control the energization of said motor in response to a signal, means responsive to the terminal voltage of said motor to reduce the sensitivity of said electron discharge means to said signal, and means responsive to the speed of said motor to reduce the sensitivity of said terminal voltage responsive means.

5. A servo system comprising input and output shafts, Selsyn transformers on said `shafts interconnected to provide an overall transformation ratio related to the difference between the angular positions of said shafts, controlled rectiflers with power input circuits connected through said transformers to a source of altemating current and control input circuits `connected to said source, whereby unidirectional voltages related in magnitude to said difference in angular positions are produced, thermionic tubes connected to be controlled by said unidirectional voltages so as to vary in impedance, amotor connected to said A..C. source through a circuit effectively including they impedances of said tubes, a mechanical connection between said motor and said output shaft, a feedback rectifier connected across said motor to provide a unidirectional voltage pro,- portional to the terminal voltage of -said motor, electrical connections between said feedback rectifier and said controlled rectiers whereby the output of said feedback rectifier opposes in effect the output of said controlled rectifiers, a resistor connected in the circuit of said feedback rectifier so as to decrease the output of said rectifier, and

9 Speed responsive switch connected across said servo systemto the diierence between said first ariel said second signals, and controlling the effect of said-.second signal in accordance with the speed of operation of a component of said servo system.

ROBERT N. LESNICK.

` Patented Nev. 21, 1944 UNiTED ST SERV() SYSTEM Robert N. Lesnick, Camden. N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 29, 1942, Serial No. 452,757

7 Claims.

This invention relates to servo systems, and particularly to the stabilization of electrically controlled systems for operating a load or output shaft to an angular position corresponding to that of a movable control or input shaft. Systems of this type generally operate by deriving a control voltage related to the difference in the positions of the two shafts and using this voltage to control a driving motor so as to move the output shaft to a position corresponding t that of the input shaft.

When the control effect is a function only of the relative angular displacement of the input and output shafts, faulty operation of the system is likely to occur due to low or zero energization of the driving motor when the two shafts are in or near positional agreement. Mere increase in the the effect of the displacement signal while `the motor is running, and may overcome the displacement signal completely while the motor is decelerating. When the motor is just starting, the velocity signal, being delayed has no effect and allows the full displacement signal to operate initially. Thus, by'using relatively high amplification of the displacement signal and timek delayed negative feedback of the velocity signal, the accuracy of the system is improved and the tendency to hunt ls reduced.

Delayed velocity feedback is not, however, entirely successful, particularly if the velocity of the control shaft is widely variable, because different feedback ratios are required for ideal operation at different velocities. This difficulty is overcome in the present invention by controlling the ratio of the velocity feedback in response to the velocity itself; the feedback is decreased after the driving motor has attained a predetermined speed, independently of the time elapsed after the initial movement of the control shaft. Similarly, the feedback is increased when the motor speed falls below a. predetermined value, regardless of the rate at which the motor decelerates to that Accordingiy, it is an object of this invention to provide an improved method of and means for stabilizing electrically controlled servo systems.

Another object is to provide an improved method of and means for introducing a velocity component in the motor control signal of a servo system.

A further object is to provide improved means for ensuring proper stabilization of a servo system over a wide range of velocity.

Other and incidental objects will become apparent to those skilled in the art upon consideration of the following description, with reference to the drawing, of which Fig. 1 is a schematic representation of a servo system embodying the invention, and Fig. 2 is a circuit diagram of an amplifier and power control device.

Referring to Fig. l, a reversible electric motor i, which may for example be an induction motor, is connected to a load diagrammaticaly represented by an arrow 3. The motor i is provided with a centrifugal switch 5, which is arranged to open when the motor speed exceeds a predetermined valu-e. Also connected to the load, by a shaft schematically indicated by the dashed line l, is a Selsyn transformer 9. The stator winding o the transformer 8 is connected to the stator of a transformer il, which may be identical to the device 9. The rotor of the transformer il is mechanically connected to a control shaft i3, which may be provided with a hand wheel i5 for manual operation. The switch 5 may be connected to the shaft I3 rather than the output shaft, if desired.

The rotor of the transformer 9 ls electrically connected Ito an alternating current supply, not shown, and the rotor of the transformer l i is connected to an amplifier and power control device i1. Referring to Fig. 2, the device Il may comprise a grid controlled rectifier or balanced modulator 25, an amplifier 21, and a power control stage 29 connected between the motor i and the A.C. supply. A feedback circuit, comprislng the switch 5, resistors i! and 2l, and a rectier 23 is connected across one winding of the motor I and to the control device l1. A pair of transformers 3i have their primaries connected between anode and cathode of the tubes 29, and secondaries connected in a. bridge circuit. One winding of the motor i is connected across one diagonal of the bridge, and the other winding is connected in series with the A.C. line to the other diagonal. A capacitor 33 is shunted across one of the motor windings to provide two phase operation. A conventional power rectifier 35 is 

